1. Field of the Invention
The present invention relates to the use of fluorinated solvents and solvent blends for processing fluorinated photopolymers such as photoresists. Such solvents and photopolymers are particularly useful for patterning organic electronic and biological materials.
2. Discussion of Related Art
Organic electronic devices can offer significant performance and price advantages relative to conventional inorganic-based devices. As such, there has been much commercial interest in the use of organic materials in electronic device fabrication. For example, organic materials such as conductive polymers and organic semiconductors can be used to manufacture devices that have reduced weight and drastically greater mechanical flexibility compared to conventional electronic devices based on metals and silicon. Further, devices based on organic materials are likely to be significantly less damaging to the environment than devices made with inorganic materials, since organic materials do not require toxic metals and can ideally be fabricated using relatively benign solvents and methods of manufacture. Thus, in light of these superior weight and mechanical properties, and particularly in light of the lowered environmental impact in fabrication and additionally in disposal, electronic devices based on organic materials have the potential to be less expensive than devices based on conventional inorganic materials.
Fabrication of electronic devices, whether from organic or inorganic materials, requires the creation on an industrial scale of precisely defined patterns of the organic or inorganic active materials in these devices, often at a microscopic level. Most commonly, this is accomplished by “photolithography,” in which a light-sensitive “photoresist” film that has been deposited on a substrate is exposed to patterned light. Although this can be done in numerous ways, typically a microscopic pattern of light and shadow created by shining a light through a photographic mask is used to expose the photoresist film, thereby changing the chemical properties of the portions of the photoresist that have been exposed to light. In a “positive” photoresist, the portions of the photoresist that are exposed to light become soluble in the “developer” solution that is then applied to the exposed photoresist, and the light-exposed portions of the photoresist are washed away (“developed”) by the developer solvent to leave a pattern of unexposed photoresist and newly exposed underlying substrate. A “negative” photoresist is treated as for a positive photoresist; however, in a negative photoresist, it is the unexposed rather than the exposed portions of the photoresist that are washed away by the developing step.
In a standard process, the photoresist material is laying on top of an active material layer that is to be patterned. Once the development has taken place, the underlying layer is etched using either a liquid etchant or a reactive ion plasma (RIE) with the appropriate etch chemistry. In either case, the photoresist layer blocks the etching of active material directly beneath it. Once the etching is complete, the resist is typically stripped away, leaving the pattern of active material on the substrate.
Alternatively, the photoresist can be used with a so-called “liftoff” technique. In this case, the resist is processed on a substrate before the active material layer is deposited. After the photoresist pattern is formed, the active material is deposited on both the substrate and the photoresist. In an additional “lift-off” or “stripping” step, remaining photoresist along with an overlying layer of active material is removed via the appropriate solvent to leave the desired patterned active material.
Although the use of photoresists is routine in traditional electronic devices based on inorganic materials, photolithography has been difficult to achieve for devices using organic materials, thereby hindering the development of devices based on these materials. Specifically, organic materials are much less resistant to the solvents that are used for conventional photolithography, as well as to the intense light sources that are sometimes used in these processes, with the result that conventional lithographic solvents and processes tend to degrade organic electronics. Although there have been various attempts to overcome these problems, e.g., by ink-jet printing or shadow mask deposition, these alternative methods do not produce the same results as would be obtained with successful photolithography. Specifically, neither ink-jet printing nor shadow mask deposition can achieve the fine pattern resolutions that can be obtained by conventional lithography, with ink-jet printing limited to resolutions of approximately 10-20 μm and shadow mask deposition to resolutions of about 25-30 μm.
US 2011/0159252 discloses a useful method for patterning organic electronic materials by an “orthogonal” process that uses fluorinated solvents and fluorinated photoresists. The fluorinated solvents have very low interaction with organic electronic materials. WO 2012/148884 discloses additional fluorinated material sets for orthogonal processing.
Although these disclosures demonstrate good progress, the disclosed systems have yet to be commercially adopted. Further improvements in orthogonal, fluorinated photopolymer systems are needed with respect to performance, robustness and cost.